1. Field of the Invention
The present invention relates to a method for manufacturing a soft magnetic powder material for preparing a soft magnetic material to be used as a core material, and the like, of a solenoid actuator and a transducer. Particularly, the present invention relates to a method for forming oxide layers with high electrical resistance at surfaces of a Fe-base soft magnetic powder.
2. Description of the Related Art
For example, a core material of the actuator is required to have high saturation magnetic flux density and high magnetic permeability, in order to increase a response speed of a solenoid valve of an internal combustion engine. The soft magnetic material to be used for the application is made by employing a cheap Fe-base soft magnetic powder with high saturation magnetic flux density as a raw powder, and sintering the powder. During the step, it is necessary to form a grain boundary segregation layer with high electrical resistance in a sintered structure, and make a sintered material with high magnetic permeability and high strength, in order to reduce a loss based on an eddy current. Hence, in recent years, technologies have been researched to manufacture a soft magnetic material, by sintering a press-molded material of a soft magnetic powder material with insulating films formed on surfaces of the soft magnetic powder, for a purpose of increasing magnetic permeability, decreasing Fe loss, and the like of the soft magnetic material.
For example, in a manufacturing method described in Japanese unexamined patent publication No. 05-036514 (pages 2, 3 etc.), Ni—Zn ferrite thin layers of a soft magnetic material are formed at surfaces of the soft magnetic powder, by initially adsorbing a metal ion by submerging an atomized Fe-base alloy powder in an aqueous solution of NiCl2 and ZnCl2, and then carrying out a ferritizing reaction via oxidation in air. Further, a magnetic composite powder is prepared, by sputtering Al in an atmosphere of nitrogen gas to form an AlN based insulating film on the Ni—Zn ferrite thin layer. After that, a molding material is obtained by adding a B2O3 powder to the magnetic composite powder and, after being press-molded in a desired shape, it is sintered at 1000 degrees Celsius under pressure by a hot press method.
However, in the manufacturing method described above, it costs much in time and effort, at a forming step of the soft magnetic Ni—Zn ferrite thin layers at surfaces of the atomized Fe-base alloy powder, and a forming step of the insulating film, by sputtering Al in an atmosphere of nitrogen gas. If a crack is caused in the insulating film, an isolating property between particles of the magnetic powder decreases, and Fe loss at the sintered soft magnetic material (loss based on an eddy current) increases. Alternatively, in a case that the insulating film is formed thickly to prevent the insulating film from cracking, there is a problem that a density of the magnetic material in the soft magnetic material decreases, the saturation magnetic flux density decreases, and magnetic properties become deteriorate.